The present invention relates in general to monitoring pH levels in human tissue (such as ischemia in cardiac tissue), and, more specifically, to a microsensor needle adapted to in-situ pH measurement in human tissue.
It is known in the field of cardiac surgery that the pH of heart tissue can be indicative of conditions in which the tissue receives insufficient oxygen. If the heart does not have sufficient oxygen, ischemia can occur. Measurement of the pH can be performed using an optical sensor having a material which fluoresces in accordance with the pH of the environment into which the florescent material is immersed (e.g., blood or tissue). For example, U.S. Pat. No. 4,798,783 to Yafuso et al discloses a micro pH sensor providing a dye material at the end of an optical fiber. Excitation light is transmitted down the optical fiber to the dye material which is selected to either fluoresce or to specifically absorb the excitation light. The ionic content of the fluidic environment into which the dye material is immersed affects the florescent or absorbing properties of the material. Light from the dye material travels back up the optical fiber to a detector for characterizing the pH. Since the excitation properties of the material also depend upon the temperature, a thermistor or other temperature sensor is typically included in the sensor probe. The emitted/returned light and the temperature are utilized by conventional algorithms to determine a pH value.
Prior art micro-sensors have typically employed glass probes. Such probes are relatively expensive and, even though they are smaller than some other types of probes, are still sufficiently large to require the creation of a passage through heart tissue having a size that can cause cellular damage. Moreover, existing probe designs have been difficult to sterilize and have had a relatively short shelf life. There is a need in the art to provide a device and method for manufacturing the device which is capable of measuring pH of tissue during cardiovascular surgery or other interrogation of human tissue that is cost effective, has a relatively long shelf life, is easy to use and sterilize, and reduces damage to tissue. Moreover, there is a need to provide a microsensor that is easily placed in a wide variety of tissue types and locations.